Once you have decided on the theme your novel will deal with, you must define your personal perspective, the view of the author. If the theme is just a word that contains the essence of the novel, the premise is a sentence that establishes your stance on the theme. It is not supposed to be a complex sentence, but a premise contains all that goes on in a story, without the characters, scenes or dates, just the theme, conflict, and conclusion.
For example, if your theme is “lies”, then your premise could be something like “ lies always end up being discovered” and the story that you’ll write will have to deal with this premise, it will be the compass, a promise you’ll make to the reader, and that you’ll have to keep.
Being a personal and subjective stance, the premise does not need to be a universal truth. It is your opinion on the theme, your thesis, which you’ll demonstrate in the novel.
Why a main premise? Because as you may have guessed, in complex stories, such as novels, there will be subplots, sub-themes, and premises for each sub-theme. The important thing at this point, is to define the main premise that will constitute your opinion on the main theme.